test: add Python 3.5 and 3.6 tests on Linux

PR-URL: #1909
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: João Reis <reis@janeasystems.com>

.travis.yml

@@ -41,6 +41,14 @@ matrix:
       python: 3.7
       env: NODE_GYP_FORCE_PYTHON=python3 EXPERIMENTAL_NODE_GYP_PYTHON3=1
       before_install: nvm install 10
+    - name: "Node.js 12 & Python 3.5 on Linux"
+      python: 3.5
+      env: NODE_GYP_FORCE_PYTHON=python3 EXPERIMENTAL_NODE_GYP_PYTHON3=1
+      before_install: nvm install 12
+    - name: "Node.js 12 & Python 3.6 on Linux"
+      python: 3.6
+      env: NODE_GYP_FORCE_PYTHON=python3 EXPERIMENTAL_NODE_GYP_PYTHON3=1
+      before_install: nvm install 12
     - name: "Node.js 12 & Python 3.7 on Linux"
       python: 3.7
       env: NODE_GYP_FORCE_PYTHON=python3 EXPERIMENTAL_NODE_GYP_PYTHON3=1